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one foot is now more difficult – and
this is what we look at our
instruments with!
Accommodation is the change in
refraction of light as it enters the eye,
248 Canadian Private Pilot Studies
caused by altering the curvature of
the lens.
The major causes of defective vision
are:
·  Hypermetropia – where the
eyeball is too short, and images
focus behind the retina (log
sight). Requires a convex lens.
·  Myopia – where the eyeball is
too long, and images focus in
front of the retina (short sight).
Needs a concave lens.
·  Presbyopia – the lens hardens,
leading to hypermetropia
(comes with old age).
·  Cataracts – the lens becomes
opaque.
·  Glaucoma – pressure inside the
eyeball.
·  Astigmatism – unequal curvature
of the cornea or lens.
Laser surgery, by the way, only
corrects one aspect of vision, that is,
long or short sight, but not both.
Rods and Cones
The retina consists of light
sensors (actually, neurons)
which are called rods and cones,
because of the way they are
shaped. Each is more efficient
than the other in different kinds
of light. The point where the
optic nerve joins the retina is
mostly populated with cones,
which work best in daylight and
become less effective at night,
or where oxygen levels are
reduced (which is significant for
smokers, whose blood has less
oxygen carrying capacity), so
you get a blind spot in the direct
field of vision, which is why you
see things more clearly at night
if you look slightly to the side of
what you want to look at. Rods
cannot distinguish colours,
either, which is why things at
night seem to be in varying
shades of grey (you see colours
simply because the vibrations
they give out are strong enough
to wake the cones up).
The more your iris is open, the
less depth of field you have, so in
darkness it is difficult to see
beyond or before a certain
distance, and you may require
glasses to help (the depth of
field in photography is an area
between the camera and the
subject in which everything is in
focus. The wider the aperture,
or iris, the shorter this distance
is, and vice versa).
The retina contains enormous
amounts of vitamin A, which is
necessary for adapting between
light and darkness. Too little
vitamin A could therefore result
in night blindness. The changeover
from light to dark takes about
20-30 minutes and should
always be allowed for when
night flying (actually, the cones
take 7 minutes, and the retina
can take up to 45 minutes).
The greatest visual acuity (that
is, the ability to see small detail)
is obtained by cones in the fovea,
in the central part of the retina,
so you must look directly at an
object to see it best. At night,
look slightly to one side,
because the rods that are
sensitive to lower levels are
outside the fovea, at the
peripheral of the retina.
Human Factors 249
With distance, objects on the
retina become smaller.
Refraction
The transparent part of the
sclerotic is known as the cornea,
behind which is the lens, whose
purpose is to bend light rays
inwards, so they focus on the
retina. If this happens in front
of it, short sightedness results. You
get long sightedness with the
point of focus behind the retina.
Both conditions cause blurred
vision, correctable by glasses,
that vary the refraction of the
light waves until they focus in
the proper place.
Blurred vision can also be
caused by stress, causing
nervous tension, and excessive
eye muscle activity leading to
eyestrain. Just relaxing often
helps this condition.
70% of light is refracted by the
cornea, and 35% by the lens.
Optical illusions
Searching for an object in a
swimming pool is difficult,
because the light rays bend as
they pass the surface and the
object appears to be displaced.
Similarly, rain on a windscreen
at night gives the impression
that objects are further away. A
good fixed wing example of an
optical illusion is a wider
runway tending to make you
think the ground is nearer than
it actually is; a narrow runway
delays your reactions, possibly
leading to a late flare and early
touchdown. Any object (like a
runway) that you think is
　
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